I hope this is the correct place to send this to. I was trying to build OpenSSH on a HP/UX 10.20 system and I got an error message. The Ansi C compiler did not understand the uint64_t stuff. I did some investigating and it seems that its a problem in how the compiler is being called. Your code ran: cc -Aa -D_HPUX_SOURCE According to the manpage this combination does not produce 64 bit ints. You need to add the +e option or run it with the -Ae option. I changed the makefile and it went ok. Well, I still had to find a replacement for /dev/urandom but after I got that going it went ok. The final result for me was: cc -Ae -D_HPUX_SOURCE I also added +O4 to get it optimized for my case as I do not expect to be runnign dbx/gdb/whatever here. That though of course has nothing to do with +e. PS: gcc failed too. Although GCC does not need the +e option, it still does not read the typedefs to set uint64_t and int64_t to long long. Those are only set if the includes have the correct set of defines.